Atomic force microscope

ABSTRACT

This invention is an atomic force microscope having a digitally calculated feedback system which can perform force spectroscopy on a sample in order to map out the local stiffness of the sample in addition to providing the topography of the sample. It consists of a three-dimensional piezoelectric scanner, scanning either the sample of a force sensor. The force sensor is a contact type with a tip mounted on a cantilever and a sensor to detect the deflection of the lever at the tip. The signal from the sensor goes to an A-D convertor and is then processed by high-speed digital electronics to control the vertical motion of the sample or sensor. In operation, the digital electronics raise and lower the piezoelectric scanner during the scan to increase and decrease the force of the tip on the sample and to use the sensor signal to indicate the change in height of the tip to measure the which is the spring constant of the sample. This constant can be determined with nanometer spatial resolution. At the same time, the instrument can determine the topography of the sample with nanometer resolution. In an alternate embodiment, the lever is connected to a separate piezoelectric driver to vary the force on the tip. This improved AFM can also be used to periodically reset the force at which the tip contacts the sample and quickly replace the tip on the sample in the event that the tip loses contact with the surface.

This is a continuation of application Ser. No. 07/831,876, filed on Feb.6, 1992 and now U.S. Pat. No. 5,224,376 which is a continuation ofapplication Ser. No. 07/707,292, filed May 30, 1991 and now U.S. Pat.No. 5,237,859 which is a continuation of application Ser. No.07/447,851, filed on Dec. 8, 1989 and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to atomic force microscopes and, moreparticularly, in an atomic force microscope having a sample holder and aprobe with a sensing tip, scanning means for creating relative movementbetween a sample in the holder and the sensing tip in orthogonal X, Y,and Z coordinate directions, sensing means for sensing the position ofthe sensing tip, and feedback means connected between the sensing meansand the scanning means for creating a scan of the tip over a sample inthe holder and for maintaining the tip in a pre-established relationshipwith respect to a scanned surface of the sample in the Z direction toobtain height information about the scanned surface of the sample, tothe improvement to also allow material characteristics information to beobtained simultaneously about the scanned surface of the sample whereinthe feedback means comprises, analog-to-digital conversion means forobtaining an analog signal from the sensing means and for converting theanalog signal into a digital signal; digital computation means includingprogram means for receiving the digital signal from theanalog-to-digital conversion means and for calculating and outputtingfirst digital vertical control signals which create desired relativemovement between a sample in the holder and the sensing tip in the Zcoordinate direction which maintain the tip in the pre-establishedrelationship with respect to the scanned surface of the sample in the Zdirection; and, first digital-to-analog conversion means for receivingthe first digital vertical control signals from the digital computationmeans and for outputting analog control signals to the scanning means toaffect movement thereof in the Z direction.

The atomic force microscope is a device which uses a very sharp tip andlow forces to profile the surface of a sample down to atomic dimensions.Such a device, using a tunneling current sensor, is described in U.S.Pat. No. 4,724,318 by Binnig et al. An improved microscope which canoperate with the tip in a fluid is described in a co-pending applicationby Hansma and Drake, Ser. No. 322,001, filed Mar. 13, 1989, now U.S.Pat. No. 4,935,634, and entitled ATOMIC FORCE MICROSCOPE WITH OPTIONALREPLACEABLE FLUID CELL, which is licensed to the assignee of thisapplication.

Basically, these devices have a tip on a flexible lever with thevertical position of the tip sensed by a detector. These detectors varyand have in the past been tunneling tips, optical beam deflection, oroptical interferometers. Other sensors such as capacitive and inductiveproximity detectors are possible. The principle of these microscopes isto scan the tip over the sample while keeping the force of the tip onthe surface constant. This force is kept constant by moving either thesample or tip up and down to keep the deflection of the lever constant.In this way the topography of the sample can be obtained from thisvertical motion and this data can be used to construct 3-dimensionalimages of the topography of the surface. In previous atomic forcemicroscopes, an analog feedback circuit varied the height of the tip orsample using the deflection of the lever as an input.

Measuring the topography of a sample does not indicate the species ofobject on the surface. For instance, when measuring a biological sampleit would be useful to measure the stiffness of the sample to separate,say, salt crystals from DNA or to separate the DNA from a hard surfacesuch as glass that it may be lying on. Topography only measures shape,not stiffness.

Wherefore, it is the object of this invention to provide an atomic forcemicroscope and method of operation which has the ability to map out boththe local stiffness and the topography of a sample with nanometerresolution in order to better distinguish features in the data.

Other objects and benefits of the invention will become apparent fromthe description which follows hereinafter when taken in conjunction withthe drawing figures which accompany it.

SUMMARY

The foregoing object has been achieved in an atomic force microscopehaving a sample holder and a probe with a sensing tip, scanning meansfor creating relative movement between a sample in the holder and thesensing tip in orthogonal X, Y, and Z coordinate directions, sensingmeans for sensing the position of the sensing tip, and feedback meansconnected between the sensing means and the scanning means for creatinga scan of the tip over a sample in the holder and for maintaining thetip in a pre-established relationship with respect to a scanned surfaceof the sample in the Z direction to obtain height information about thescanned surface of the sample, by the improvement of the presentinvention to also allow material characteristics information to beobtained simultaneously about the scanned surface of the sample whereinthe feedback means comprises: analog-to-digital conversion means forobtaining an analog signal from the sensing means and for converting theanalog signal into a digital signal; digital computation means includingprogram means for receiving the digital signal from theanalog-to-digital conversion means and for calculating and outputtingfirst digital vertical control signals which create desired relativemovement between a sample in the holder and the sensing tip in the Zcoordinate direction which maintain the tip in the pre-establishedrelationship with respect to the scanned surface of the sample in the Zdirection; and, first digital-to-analog conversion means for receivingthe first digital vertical control signals from the digital computationmeans and for outputting analog control signals to the scanning means toaffect movement thereof in the Z direction.

In the preferred embodiment, there is second digital-to-analogconversion means for receiving second digital control signals from thedigital computation means and for outputting analog control signals tothe scanning means to affect movement thereof in the X and Y directionswherein the digital computation means further includes program means forcalculating and outputting the second digital control signals whichcreate a raster scan movement between a sample in the holder and thesensing tip in the X and Y coordinate directions. Preferably, the sampleholder is attached to the scanning means and the probe is heldstationary whereby the sample in the sample holder is moved with respectto the sensing tip to create the relative movement between the two.

Also in the preferred embodiment, the digital computation means includesmeans for modulating the relative movement between the tip and scannedsurface of the sample in the Z direction so as to vary the force on thesample and for calculating the stiffness of the sample as a function ofchanges in the force and changes in the position of the tip. In oneapproach modulation by the digital computation means is done duringscanning in the X and Y directions to obtain the stiffness of the sampleas a function of X and Y. In another, force variation done by thedigital computation means is done so as to vary the average force duringeach scan line so as to obtain the stiffness of the sample as a functionof force. The force may be varied so as to obtain stiffness as afunction of force at one or more points on the sample.

In another aspect, the digital computation means includes means fordetermining a tip null point and means for then setting the verticalposition of the sample with respect to the tip so that the force betweenthe tip and the sample is a pre-determined value.

The invention may also have the digital computation means include meansfor varying the vertical position of the sample with respect to the tipso as to obtain a force position curve and means for then setting theforce of the tip on the sample near a minimum value as determined fromthe curve.

In another approach, the digital computation means includes means fordetecting that the tip has come off the surface and means to interruptthe feedback process to place the tip quickly back on the surface inorder to continue the scan with little loss of data.

In an alternate embodiment, the atomic force microscope of the inventionadditionally comprises supplemental positioning means for applying asupplemental force to the tip in the Z direction wherein the digitalcomputation means includes means for sending control signals to thesupplemental positioning means whereby to set the force of the tip onthe sample.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of one implementation of an atomicforce microscope according to the present invention.

FIGS. 2-4 are simplified drawings depicting one way of operating theatomic force microscope of FIG. 1 according to the method of thisinvention.

FIGS. 5 and 6 are simplified drawings depicting alternate ways ofoperating the atomic force microscope of FIG. 1 according to the methodof this invention.

FIGS. 7 and 8 are simplified drawings depicting yet another way ofoperating the atomic force microscope of FIG. 1 according to the methodof this invention when modified by the addition of a supplementalpiezoelectric positioner.

FIG. 9 is a simplified drawing depicting still another way of operatingthe atomic force microscope of this invention.

FIG. 10 and 11 are simplified drawings depicting an alternate way ofoperating the atomic force microscope of this invention.

FIG. 12 is a graph of tip vertical position versus sample verticalposition when operating the atomic force microscope of this inventionaccording to one of the possible variations of the method thereof.

FIG. 13 is a drawing of a way to operate the atomic force microscope ofthis invention to minimize loss of data.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The atomic force microscope of this invention and its method ofoperation which are now to be described in detail, can map out both thelocal stiffness and the topography of a sample with nanometer resolutionin order to better distinguish features in the data. FIG. 1 shows ablock diagram of one implementation thereof wherein the atomic forcemicroscope (AFM) is generally indicated as 10.

The feedback loop, generally indicated as 12, of the AFM 10 is unlikepriorart feedback loops as applied to atomic force microscopes whereinthe feedback only positions the tip of the probe at a constant positionwith relationship to the surface of the sample--yielding onlytopographic data.The feedback loop 12 of this invention is controlled byhigh speed programmable electronics 14 which accepts the sensor signalfrom the tip sensor 16, which has been converted to digital form by oneor more analog-to-digital (A-D) converters 18. This signal at 20 isprocessed digitally by the high speed programmable electronics 14 andthe outputs 22and 24 thereof control the (X, Y) and Z scanning,respectively, of the sample 26 which, in this embodiment, is carried onthe X, Y, Z piezoelectric positioner 28, which is of a type well knownto those skilled in the art. The digital outputs from the high speedprogrammable electronics 14 on outputs 22, 24 are connected to thepositioner 28, whichis driven by analog voltages applied thereto,through one or more digital-to-analog (D-A) converters 30. Such apositioner 28 could be a piezoelectric tube which deflects when voltagesare applied to electrodes on it (not shown) to produce three-dimensionalmotion of the sample 26. Inan alternate arrangement, the sample 26 couldbe stationary with the sensing tip 32 mounted on the positioner 28 for3-dimensional movement thereby, or the X-Y motion could be applied tothe tip (sample) and the Z motion to the sample (tip). The AFM 10 ofthis invention (in either embodiment--probe or sample moving) is morecomplex than the conventional atomic force microscope; but, is able tomeasure not only topography but also local stiffness as a function offorce. It can also set periodically and very sensitively the forcebetween the tip and sample, and calibrate the force between the tip andsample to give absolute values to the stiffness. It can also be operatedto minimize loss of data when scans arebeing made at very low tip-sampleforces.

In a pre-production commercial embodiment as built and tested by theinventors herein, the high speed electronics 14 samples the position ofthe lever 34 carrying the tip 32 25,000 times a second. It takes as aninput at 20 the lever position and then calculates and outputs at 22(through scan controller 42) and 24 the signals required to adjust thevertical position of the sample 26 in order to control the force betweenthe tip 32 and the sample 26. Those skilled in the art will readilyrecognize and appreciate that this is quite unlike conventional forcemicroscopes in which the height of the sample is adjusted by thefeedback loop and the height (as represented by the analog Z positionalvoltage into the positioner) is then read by the associated calculatingcomputer to construct an X, Y, Z topography. In the atomic forcemicroscope of thisinvention, the height of the sample is calculated bythe digital electronics 14 and then output to the piezoelectric scanningpositioner 30. In this manner, the control of the height of the sample26 can be usedfor more than just keeping the force constant; and, can beused to manipulate this force to obtain much more information about thesample 26 than can be done with a conventional system according to theprior art. The feedback calculation in the AFM 10 of this invention iscontrolled by a program contained in the program memory 36 and thisprogram can be changed by the operator through the computer 38. Thecomputer 38 also has access to the topography and stiffness data whichthe high speed electronics 14 places in the data memory 40 during or atthe end of a scan.

In the present implementation as built and tested by the inventorsherein, the sample 26 is mounted on a conventional piezoelectric tube(being the positioner 28) which is able to move the sample 26 in threedimensions by the application of appropriate control voltages thereto.The high speed digital electronics 14 consists of a digital signalprocessor having the program memory 36 and data memory 40 connectedthereto. The digital signalprocessor within the high speed digitalelectronics 14 also controls the X-Y scanning (through the scancontroller 42) so that nonlinear scan waveforms can be used for thescanning in order to compensate for nonlinearities in the piezoelectricdevice. These nonlinear waveforms are calculated by the signal processorfrom formulas which the inventors have found to describe thenonlinearities in the scanner. This method is described in our copendingapplication Ser. No. 07/622,353 filed Nov. 29, 1990 and now U.S. Pat.No. 5,051,646, by Virgil Elings and John Gurley entitled METHOD OFDRIVING A PIEZOELECTRIC SCANNER LINEARLY WITH TIME, also assigned to thecommon assignee of this invention, the teachings of which areincorporated herein by reference.

In the preferred implementation of the AFM 10, a stiffness map(stiffness as a function of the XY position) of the sample 26 isobtained by modulating the force between the tip 32 and sample 26 duringa scan by having the digital electronics 14 modulate the verticalposition of the sample 26 while also keeping the average force betweenthe tip 32 and sample 26 constant. The bending of the lever 34, which isa measure of theforce on the tip 32, is measured by an optical detector(being the tip sensor 16) which senses the deflection of a light beamwhich is reflected from the back of the lever. This approach to tipsensing in an atomic force microscope is, of course, known to thoseskilled in the art and, perse, forms no point of novelty of thisinvention. Such a detector may consist of several parts or sections,each one of which may be connected to an A-D convertor or some analogmixing of the signals (such as subtraction) may occur before the signalis digitized.

To measure the topography of the sample 26, the digital electronics 14takein the deflection of the light beam, which is a measure of thevertical position of the tip 32, and regulate the height of the sample26 so that the position of the tip 32, and hence the force of the tip 32on the sample 26, stays constant as depicted in FIGS. 2-4. Under thiscondition, the sample 26 is moved up and down by an amount which isequal to the topography of the surface and hence this motion, which iscalculated and sent to the vertical position Z, is a measure (inverted)of the topographyof the sample 26 and is stored into data memory 40.This feedback can be made to depend on any function of the input signalas determined by the program in program memory 36. The inventors hereinnormally use proportional-integral feedback; but, have also used a verynonlinear feedback gain which insures that the tip 32 will not bedamaged on the surface of the sample 26.

Another way of operating the microscope is to not move the sample in thevertical direction with feedback; but, to measure the topography fromthe deflection of the lever. This method of operation is shown in FIGS.5a and5b where the sample is scanned under the tip at constant heightand the deflection ΔZ of the lever is used as a measure of thetopography. The disadvantage of this mode of operation is that the forceof the tip onthe sample varies as the lever deflects and this may affectthe topography or result in unnecessarily large forces.

The digital electronics can also be used to modulate the verticalposition of the sample 26 as shown in FIGS. 6a and 6b. This modulationis typicallyrapid compared to the scanning rate across the sample 26. Asthe sample 26 is modulated up and down by an amount ΔZ, the lever 34will deflect by some amount ΔZ'. For an infinitely stiff sample 26, thelever 34 will deflect by the same amount the sample 26 is moved, i.e.ΔZ'=ΔZ. For a sample 26 with finite stiffness, the sample 26 will deformsomewhat and ΔZ' will be less than ΔZ. If the spring constant of thelever 34 (ΔF/ΔZ') is equal to K', the modulation in force is ΔF=K'ΔZ'.The deformation of the sample26 is Δ=ΔZ-ΔZ', so the spring constant ofthe sample 26 is:

    K=ΔF/Δ=K'ΔZ'/(ΔZ-ΔZ')=K'/(ΔZ/ΔZ'-1)

This quantity can be determined at every data point along the scan sothat both topography and stiffness can be determined as a function of Xand Y, the horizontal coordinates along the sample 26. Of course, onecould imageinverse stiffness or simply use ΔZ' or Δ as an indicator ofstiffness. The term "stiffness" is used herein to mean any parameterwhichis a measure of how the sample deforms with increasing force.

It should be noted that one can program the digital electronics 14 tomodulate the vertical position more than once at each topographical datapoint in order to obtain an average so as to reduce random noise in thesignal. It should be noted as well that although the vertical positionof the lever 34 is being modulated, this need not confuse the feedbackcalculation since the feedback can operate from the average verticalposition of the lever 34; or, alternately, from the high or low point ofthe modulation. In a conventional prior art AFM system with analogfeedback, any attempt to modulate the sample 26 is canceled by thefeedback loop which is trying to keep the lever 34 at a constant height;so, one must modulate at a frequency beyond the response of the feedbackloop, which gives one little choice in operating parameters. Such priorart AFMs do not have the flexibility of the AFM 10 of this invention.

The inventors herein have already used this improved AFM and forcespectroscopy to locate atoms on the surface of a mica sample and tostudy DNA laying on a glass surface. The method of modulating the forceand thenimaging the stiffness of the sample has the advantage that asurface such as glass, which has a rough topographic image, will have aflat stiffness image and therefore soft molecules on it such as DNA canbe easily imaged and studied.

In an alternate configuration, as shown in FIGS. 7 and 8, a separatepiezoelectric driver 44 is connected to the lever 34 through a D-Aconverter 30 from the high speed electronics 14 as a source ofindependentand controlled force modulation to the tip 32; but now, in away that for astiff sample 26 there is no motion of the lever 34. Forexample, let us saythat for the tip 32 not on a sample 26 as depicted inFIG. 7, this piezoelectric driver 44 moves the lever 34 up and down byan amount ΔZ_(o). For a lever 34 spring constant of K', the forceapplied bythe tip 32 on a stiff sample 26 would be ΔF=K'ΔZ_(o). On anelastic sample 26, this force would cause the tip 32 to deform thesample 26 by ΔZ', as shown in FIG. 8. The force now would be K'(ΔZ_(o)-ΔZ') and the stiffness of the sample 26 would be:

    K=ΔF/ΔZ'=K'(ΔZ.sub.o -ΔZ')/ΔZ'

    K=K'((ΔZ.sub.o /ΔZ')-1)

This would be more accurate on stiff samples where K>K' whereas theprevious implementation would be more accurate for soft samples whereK<K'.

The average force of the tip 32 on the sample 26 can be varied during ascan or from scan line to scan line so that this stiffness could bemappedat different force levels. One could use the AFM 10 of thisinvention to see whether this stiffness is linear or nonlinear in orderto determine more about the sample 26. One method of operating would beto scan only inthe X direction, i.e. over the same line with no Ymotion, and increase theaverage force during each line scan byprogressively increasing the deflection of the lever 34 by incrementingthe tip 32 position at which the feedback holds constant. This wouldgive a map of stiffness versus force on the sample 26 for one scanacross the sample 26. This mode of operation is depicted in FIG. 9.

Another mode of operation is at each point or at selected points tomeasurea stiffness versus force curve for the sample 26. This would bedone by having the digital electronics 14 modulate the vertical positionof the sample 26 while the sample 26 is, on the average, being movedupward (downward) to increase (decrease) the force of the tip 32 on thesample 26. This mode of operation is depicted in FIGS. 10 and 11.

The digital electronics 14 controlling the vertical position of thesample 26 is used to calibrate and set the force with which the tip 32presses onthe surface of the sample 26. FIG. 12 shows a typical tipdisplacement versus sample displacement curve as the sample 26 is raisedtoward the tip32. Initially, with the tip 32 not in contact with thesample 26, the forcebetween the tip 32 and sample 26 is zero at point Aand then becomes attractive, pulling the tip 32 down toward the surfaceat point B, due to the attractive Van der Waals force. At Point C, thisattractive force overcomes the spring tension of the lever 34 and thetip 32 then drops to the surface of the sample 26 until the contactprovides a repulsive force between the tip 32 and sample 26. As thesample 26 is raised further, the tip 32 now follows the sample 26 upwardwith the tip 32 remaining in contact with the sample 26 along the pathportion D, E and F. The slope ofthis path will depend somewhat on thestiffness of the sample 26.

When the sample 26 is retracted, the tip 32 follows the sample 26 beyondthe point where the tip 32 touched the sample 26 during the upwardmotion if there is moisture on the surface which holds the tip 32 on thesurface by surface tension. At point G, the tip 32 breaks free from thesurface. As seen in the figure, the total tip 32 force can be positiveas in regionF, zero, as at E, or negative as at B and D.

For very sensitive samples, it is necessary to operate sometimes withthe lowest possible repulsive force on the tip 32, just at the pointwhere thetip 32 wants to break loose from the sample 26. This would bethe setpoint force about which the feedback calculations would operateto hold the force constant. Because of drifts in the sensor 16 and creepin the piezoelectric scanning positioner 28, this setpoint tends todrift during operation so it is important that the control system canactivate the setpoint procedure at various times, say at the end of acomplete scan frame or even at the end of each scan line. This isimportant for biological samples where very small forces are required.This was pointed out by Weisenhorn et al in "Applied Physics Letters",54(26) Jun. 26, 1989, p 2651. In their apparatus, however, they wouldneed to interrupt the data-taking to remove the feedback loop andperform the force calibration (for reasons associated with prior artanalog feedback as discussed earlier herein). By contrast, in order tofind the operating point, the digital electronics 14 of the AFM 10 ofthis invention, under program control, can vary the height of the sample26 to determine this curve, from the tip sensor input, and set the forcebetween the tip 32 andsample 26.

Since the spring constant K' of the lever 34 is known from its geometry,measuring the deflection of the lever 34, ΔZ, from its null point Agives the force between the tip 32 and sample 26 by the formula F=K'ΔZ.The calibration procedure can quickly find the null point and check theforce displacement curve to assure that the instrument is operatingwell.

When operating with very low forces between the tip and sample, priorart devices have had the problem that the tip will leave the surface,causing the entire image to be lost, since the prior art feedback willnot bring the tip back down on the surface. The problem is thefollowing: The force applied to a sample is comprised of long-range Vander Waals force, which attracts the sample to the tip, and a short rangecoulomb force which is repulsive. The short range coulomb force at thetip is concentrated over avery small area and can adversely affect softsamples by causing them to rupture. It is therefore desirable tominimize the coulombic force at the tip for imaging soft samples. Whenthe feedback set point is adjusted to minimize the repulsive coulombforce, the net force on the cantilever is attractive, displacing itdownward, such as point D in FIG. 12. While running in the attractivemode, the prior art feedback loops were unstable. Any bump on thesurface or noise would cause the tip to leave the surface. Since the netdeflection is negative, the tip springs to the null point at a locationabove the set point. Thus, conventional feedback interprets this as arise in the surface and moves the sample down, which continues pullingthe tip and sample apart, i.e. the feedback is now in the wrongdirection. The tip will not go back to the surface until this feedbackis interrupted and the tip reset.

In prior art systems, the lowest tip force that could be used was set bythe lowest force possible to complete an entire scan without the tipleaving the surface. On rough surfaces, this minimum force was limitedby the above-described effect and could be larger than desired.

The improved atomic force microscope of this invention can produceimages at lower tip force by being able to bring the tip back down tothe surfacewhenever it comes off and continue imaging with little databeing lost. Thedigital computation logic can be programmed to detectthat the tip has mostlikely come off the surface and then respond bygoing into a mode which brings the tip back to the surface. It is bestthat the scanning in the horizontal direction continue during thisprocess, since the hysteresis inthe scanner can be controlled better ifeach scan line is scanned in the same way.

FIG. 13 shows a method according to the present invention fordetermining if the tip 32 has most likely come off the surface. As thesample 26 is normally scanned along, the height of the tip 32 isdetected. When the tip32 goes up, the sample 26 is moved down to keepthe tip 32 at a constant height (the tip 32 will follow the sample 26).If a large positive motion of the tip 32 is detected (larger than somethreshold value), then the program will, instead of lowering the sample26, raise the sample 26, assuming that the tip 32 has come off thesample 26. The program would then check the tip 32 height. If it doesnot change, then the tip 32 is indeed off the surface and the sample 26will then be raised to contact the tip 32. This could be done in aseries of steps of moving and checking, or could be done quickly, sincethe processor could know the shape of the curve in FIG. 12 and couldbring the sample 26 up a certain amount and then lower it to get theoperating point D. The topography would be essentially known from theprevious scan line, so the program could use that information to help inbringing the sample 26 to the tip 32and keep the forces low.

So, as can be seen from the foregoing description, this improved atomicforce microscope is able to operate in a mode where the event of the tip32 leaving the surface is quickly corrected. As a further improvement,theinstrument either in the high speed electronics or computer canreplace thebad data taken when the tip 32 is off the surface with datataken on the previous scan line or the next scan line (or some average)to produce an image with little distortion due to lost data.

In summary, what has been disclosed hereinbefore is an improved atomicforce microscope which uses programmable high speed digital circuitry inthe feedback control so that the device can be operated in several modesto improve the performance of the microscope.

Wherefore, having thus described our invention, what is claimed is: 1.An atomic force microscope comprising:a sample holder and a probe with asensing tip; scanning means for creating relative movement between asample in the holder and the sensing tip in X, Y, and Z coordinatedirections; sensing means for sensing the vertical position of thesensing tip; and feedback means connected between the sensing means andthe scanning means for maintaining the tip in a preestablishedrelationship with respect to a scanned surface of the sample in the Zdirection, thereby to obtain height information about the scannedsurface of the sample, wherein the feedback means comprises,analog-to-digital conversion means for obtaining an analog signal fromthe sensing means and for converting said analog signal into a digitalsignal, digital computation means including program means for receivingsaid digital signal from said analog-to-digital conversion means and forcalculating and outputting first digital vertical control signals whichcreate desired relative movement between a sample in the holder and thesensing tip in the Z coordinate direction to maintain the tip in thepre-established relationship with respect to the scanned surface of thesample in the Z direction, and first digital-to-analog conversion meansfor receiving said first digital vertical control signals from saiddigital computation means and for outputting analog control signals tothe scanning means to effect movement thereof in the Z direction; andwherein: said digital computation means includes means for determiningthat the sensing tip has come off the surface of the sample, includingmeans for determining whether a motion of the tip in a predetermineddirection exceeds a predetermined threshold, means for signalling saidfeedback means to move the sample in said predetermined direction whenit is determined that the motion of the tip in the predetermineddirection has exceeded said predetermined threshold, means fordetermining whether or not upon said moving of said sample in saidpredetermined direction a responsive movement of said tip in saidpredetermined direction has been produced, and means for determiningthat the tip has come off the surface of the sample when it has beendetermined that said responsive movement of the tip has not beenproduced; and means for generating control signals to place the tip backon the surface of the sample.
 2. The improvement to an atomic forcemicroscope of claim 1, wherein:said digital computation means includesmeans for replacing bad data taken when the tip is off the surface ofthe sample with data from an adjacent scan line.
 3. In a method ofoperating an atomic force microscope having a sample holder and a probewith a sensing tip, wherein a relative scanning movement between asample on the holder and the sensing tip is created in X, Y, and Zcoordinate directions and the position of the sensing tip is sensed, anda feedback operation is performed to maintain the tip in apreestablished relationship with respect to a scanned surface of thesample in the Z direction based on the sensed position of the tip,thereby to obtain height information about the scanned surface of thesample, the improvement comprising the steps of:performing digitalfeedback comprising, obtaining an analog signal position from thesensing means, converting the analog signal with analog-to-digitalconversion means into a digital signal, receiving the digital signalfrom the analog-to-digital conversion means, calculating and outputting,with digital computation means, first digital vertical control signalswhich create desired relative movement between a sample in the holderand the sensing tip in the Z coordinate direction to maintain the tip inthe preestablished relationship with respect to the scanned surface ofthe sample in the Z direction, receiving the first digital verticalcontrol signals from the digital computation means, and outputtinganalog control signals to the scanning means to affect movement thereofin the Z direction; additionally comprising: a) determining that thesensing tip has come off the surface of the sample, includingdetermining whether a motion of the tip in a predetermined directionexceeds a predetermined threshold, producing control signals to move thesample in said predetermined direction when it is determined that themotion of the tip in the predetermined direction has exceeded saidpredetermined threshold, determining whether or not upon said moving ofsaid sample in said predetermined direction a responsive movement ofsaid tip in said predetermined direction has been produced, anddetermining that the tip has come off the surface of the sample when ithas been determined that said responsive movement of the tip has notbeen produced; and, b) sending control signals to the scanning means tocontrol the relative vertical position of the tip and sample to placethe tip on the sample with a minimal loss of data.
 4. The method ofoperating an atomic force microscope of claim 3, comprising the stepof:replacing bad data taken when the tip is off the surface with datafrom an adjacent scan line.